There are now quite a number of approved medical procedures that involve the ablation or removal of tissue. In particular, many of these procedures are performed by advancing a catheter through the vasculature of a patient to an operational site. Depending on the requirements for the particular procedure, the target tissue that is to be ablated may be characterized as being a single spot, a series of spots or a linear ablation (i.e. a straight line or curvilinear ablation). Further, due to the nature and the anatomical constraints that are imposed on the procedure by the vasculature, each procedure will present unique issues for consideration.
In recent years, various catheters have been proposed for the purpose of ablating tissue in diverse parts of the vasculature. Early devices focused on the use of radiofrequency (rf) energy or ultrasonic energy to ablate internal tissue. More recently, however, it has been determined that cryoablation techniques may be preferable in many applications. Insofar as cryoablation techniques are concerned, a specific issue for consideration involves the control of the necessary temperature conditions in the vasculature. On the one hand, proper temperature conditions must be created in the area where the cryoablation is to occur. On the other hand, these temperature conditions must be confined to the desired area to avoid adversely impacting non-target tissue.
The destruction of tissue by cryoablation requires that the targeted tissue be cooled below a certain temperature. In addition, recent studies have suggested that the cooling rate and subsequent warming rate can affect the percentage of tissue cells destroyed in a cryoablation procedure. For example, co-pending, co-owned U.S. patent application Ser. No. 11/050,974 filed Feb. 4, 2005 and titled “Warming Gradient Control for a Cryoablation Catheter” discloses a regimen of preferred cooling and warming rates to maximize tissue destruction. As such, U.S. patent application Ser. No. 11/050,974 is hereby incorporated by reference herein. It can be appreciated that when these methods are applied to create a linear lesion, it becomes important to ensure that the preferred cooling and warming rates are maintained uniformly along the length of the targeted tissue.
One application in which a linear ablation is currently prescribed is in the treatment of irregular heart rhythms such as atrial fibrillation. Specifically, it is believed that at least one-third of all atrial fibrillation cases are caused by irregular electrical signals that originate in one or more of the four pulmonary veins. It is further believed that the optimal technique for treating atrial fibrillation is to electrically isolate these pulmonary veins by creating a linear, circumferential lesion around each ostia where an affected pulmonary vein connects with the left atrium. To be effective, each linear conduction block must completely block all of the irregular electrical signals and this often requires the ablation of a relatively deep and long, uniform lesion.
To efficiently create a uniform linear lesion such as the one described above, it is typically desirable to simultaneously ablate all of the targeted tissue in a one-step cryoablation process. For this purpose, it is typically necessary to use an element having a contact surface that is shaped (or shapeable at the operational site) to conform to the shape of the desired linear lesion. Along these lines, co-pending, co-owned U.S. patent application Ser. No. 10/876,312 filed Jun. 24, 2004 and titled “Active System for Deflecting a Distal Portion of a Catheter into a Hoop Configuration” discloses a system for contacting and cryoablating a linear circumferential band of internal target tissue in a one-step cooling process. Accordingly, U.S. patent application Ser. No. 10/876,312 is hereby incorporated by reference herein.
When a one-step cooling process is used to ablate a linear lesion, it is often preferable that the operable contact surface be uniformly cooled along its length. In this regard, phase change refrigerants can be used to cool a contact surface by undergoing a liquid to gas phase transition in close proximity to the contact surface. For example, co-owned U.S. Pat. No. 7,004,936 entitled “A Refrigeration Source for a Cryoablation Catheter” discloses a system for delivering a liquid phase change refrigerant to a volume adjacent a contact surface for transition to a gas in the volume to cool the contact surface. Accordingly, U.S. Pat. No. 7,004,936 is hereby incorporated by reference herein. In some cases, when a phase change refrigerant is used to cool a relatively long linear contact surface, it can become necessary to vaporize the liquid refrigerant at more than one point along the length of the contact surface in order to uniformly cool the contact surface.
In light of the above, it is an object of the present invention to provide medical applicators such as catheters and probes for the purposes of cryoablating linear shaped lesions. It is another object of the present invention to provide devices and methods for uniformly cooling a linear contact surface of a contact element to a cryogenic temperature with a phase change refrigerant. Further, it is an object of the present invention to create a linear structure that provides for simultaneous liquid-to-gas phase changes at various locations along the length of the cooling segment. Yet another object of the present invention is to provide devices and methods for cryoablating linear shaped lesions that are easy to use, relatively simple to implement, and comparatively cost effective.